1. Field of the Invention
This invention relates to a crosstalk canceler, and more particularly to a crosstalk canceler which efficiently cancels crosstalk generated between neighboring tracks on an optical disc of high recording density.
2. Description of the Prior Art
There have been developed many types of high density information recording methods of an optical disc. One method of accomplishing high density recording irrespective of the size of the disc is to reduce the width of the tracks (i.e., track pitch) on the disc. The spot size of the laser beam is determined by the wavelength of the laser beam and the numerical aperture (NA) of the objective lens. Therefore, when the track pitch is reduced, the laser spot covers not only the target track but the neighboring tracks, and consequently crosstalk arises. In order to eliminate the crosstalk generated due to the reduction of the track pitch, there has been proposed a method of calculating the crosstalk level from the read-out signals of the target track and the neighboring tracks and eliminating the crosstalk to obtain the information signal of only the target track.
According to a known method of crosstalk cancellation, information signals of a target track (center track) and neighboring tracks on both sides thereof are read out by irradiating light beams. In the read-out signal of the center track, crosstalk components from neighboring tracks on both sides are mixed. The ratio of crosstalk, i.e., ratio of the signal of the neighboring track mixed into the center track is prescribed depending upon the relationship between the size of the laser spot and the track pitch and/or other factors. Therefore, by detecting the crosstalk ratio and attenuating the read-out signal of the neighboring track by the attenuation coefficients determined based on the detected crosstalk ratio, a pseudo crosstalk component can be produced. Then, the pseudo crosstalk component is subtracted from the read-out signal of the center track, thereby obtaining the true information of the target track. In this way, by determining appropriate attenuation coefficient, the crosstalk component may be effectively canceled. According to the above method, crosstalk component can be correctly eliminated even in a high density optical disc, and thereby read-out signal of high S/N ratio can be obtained. In this connection, similar crosstalk canceling techniques have been proposed in U.S. Pat. No. 5,280,466 and Japanese Patent Applications Laid-Open under Nos. 3-232118, 340225, and 5-205280.
In the above described method, it is necessary to accurately determine appropriate attenuation coefficients for attenuating the read-out signal of the neighboring tracks. In this view, the techniques proposed in the above mentioned documents set attenuation coefficients of the attenuator or filter coefficients of the filters according to the adapted arithmetic algorithm, and accomplish the arithmetic operation by hardware such as an operation circuitry. However, such arithmetic operations necessarily require a relatively large-scaled circuitry and a long operation time. This is because the arithmetic operation for calculating the attenuation or the filter coefficients needs multiplications and/or divisions. Generally, a multiplication of an N-bits digital number and an N-bits digital number using a digital circuit requires additions for (N-1) times. A division of digital number may be accomplished by repetition of comparisons and subtractions, or multiplication of the inverse of the multiplier, however, these need more complicated processing and a circuitry of larger scale. Namely, multiplication or division requires longer operation time and larger circuit scale of more than (N-1) times compared with addition or subtraction. Namely, the known crosstalk cancellation techniques require a large-scaled circuitry and complicated processing because the arithmetic operation for coefficient determination should include multiple times of multiplications and/or divisions.